[llvm] MachineScheduler: Improve instruction clustering (PR #137784)

[via llvm-commits]

ruiling wrote:

> Even after this patch, we still add weak "cluster" edges to the DAG. Is that still necessary?
> 
> Oh I see you mentioned that already. The DAG edges are only needed for some sort of check in MacroFusion, right? I don't really understand that part.

For the edge in `clusterNeighboringMemOps`, Removing it just causes more test changes. That's one of the reasons why I leave it there. I slightly prefer we remove it but no clear idea yet. I think it needs some more investigation, and it would be better done separately.

https://github.com/llvm/llvm-project/pull/137784